Method of making a pressure transducer

ABSTRACT

A low cost pressure transducer can be manufactured and assembled with automatic or semiautomatic production line techniques. The transducer includes a pressure fitting, a diaphragm, a strain gage comprising a bridge circuit and tabs with leads for the bridge circuit, and a termination board, all contained within a case. One end of the case is swaged over a flange on the fitting in order to sealingly clamp the diaphragm between a shoulder in the case and the fitting flange. The bridge portion of the strain gage is adhesively secured to the diaphragm. A pressure distribution member is placed over the bridge so pressure may be applied to the assembly during heat-curing of the adhesive which secures the gage to the diaphragm. The termination board has four terminals and a corresponding number of conductive strips. It is positioned in the case so that each of the conductive strips is pressed into electrical contact with a corresponding lead on the tabs of the gage. The other end of the case is swaged over the termination board to lock the latter in place.

This application is a division of my copending U.S. Application Ser. No.456,122, filed Mar. 29, 1974 for Pressure Transducer now U.S. Pat. No.3,899,766.

This invention relates to pressure transducers and more particularly tonovel diaphragm transducers of the resistive strain gage type and themanufacture thereof.

Pressure transducers of the type used to sense and measure fluidpressure, generally utilized a mechanical sensing element. Theseelements are relatively thin-walled elastic members, such as diaphragms,plates, shells or tubes which offer the pressure (force) a surface(area) to act upon. When the pressure to be measured is not balanced byan equal pressure acting on the opposite surface, the elastic member iscaused to deflect, producing stress and resulting strain in the element.This resulting strain can be measured by a resistive strain transducer,more commonly known as the strain gage. It generally consists of asensing element in the form of a conductor or semiconductor of smallcross-sectional area which is mounted to one surface of the elasticmember (which is more commonly in the form of a diaphragm) so that itexpands or contracts with the elastic member. This deformation of thesensing element causes it to undergo a change in resistance and thusexhibits what has been termed a piezoresistive effect. Hence, a straingage senses strain by its own deformation and transduces the deformationinto a resistance change.

The resistance change of a strain gage is usually converted into voltageby connecting one, two or four similar sensing elements as arms of aWheatstone bridge, also called a strain gage bridge. By applying avoltage to the bridge the bridge output voltage will be a measure of thestrain sensed by each sensing element. Each arm of such a bridgecontaining a strain-sensing element is referred to as an active arm.Strain transuction can, therefore, be said to be performed by the activearms of a strain-gage bridge.

Various types of strain gage transducers have been commerciallydeveloped including bare-wire bondable strain gages, bondable wirestrain gages on a paper or plastic carrier base, metal-foil bondablestrain gages, semiconductor strain gages and deposited-metal (thin film)strain gages.

In pressure transducers of the type employing strain gages mounted ondiaphragms, typically the diaphragm material will have a thickness inthe order of 0.005 inch with an effective diameter of 0.5 inch for a0-15 psi transducer. Heretofore, a great deal of skill as well ascareful procedure has been required to mount the diaphragm in a casingand to complete the assembly of the transducer so that the diaphragm andstrain gage will exhibit uniform characteristics. As a result, mostaccurate commercially available pressure transducers of the resistivestrain gage type have been difficult to assemble and, thus, relativelyexpensive to manufacture. One solution which has been suggested tosimplify the assembly procedure is to make the diaphragm and casing as asingle part. Such an approach, however, has not proven feasible since itmakes it difficult to produce a diaphragm having the desired physicalcharacteristics.

Another problem encountered with the commercially available pressuretransducers of the type utilizing metal-foil strain gages is thatexternal leads must be soldered or welded to the terminals of the straingage.

Accordingly, an object of the present invention is generally to overcomethe above disadvantages of the prior art.

Another and more specific object of the present invention is to providean improved pressure transducer which is easily and quickly assembledand thus is relatively inexpensive to manufacture.

Yet another object of the present invention is to provide an improvedinexpensive pressure transducer whose parts can be made by automatedequipment, and more importantly, whose assembly can be automated or atleast semi-automated.

Still another object of the present invention is to provide aninexpensive method for making pressure transducers of the resistivestrain gage type.

A further object is to provide pressure transducers utilizing straingages of the resistive type wherein terminal conductors are in pressurecontact with the terminal tabs of the strain gage, thereby eliminatingthe need for welding or soldering together the terminal conductors andgage tabs.

Described briefly, a transducer which is constructed in accordance withthe present invention comprises a diaphragm which is assembled between apressure fitting and a portion of a housing or case. The end of the caseis swaged over the end of the fitting in order to clamp and seal thediaphragm in place. A metal foil gage comprising an encapsulated straingage bridge as a center portion and tab portions which includeunencapsulated electrically conductive lead strips or terminal tabs isthen added to the foregoing subassembly, with the center portion of thegage being installed on an adhesive-coated side of the diaphragm. In thepreferred method of assembling the transducer device, the center portionof the gage is adhesively staked to the diaphragm and a pressuredistribution member is placed over the center portion of the gage. Thena vented termination board having a plurality of terminals and acorresponding number of conductive strips is secured to the case inspaced relation to the diaphragm so that each of the conductive stripson the board is clamped into electrical contact with a corresponding oneof the terminal tabs. The diaphragm is then subjected to a balancedpressure and the assembly heated for a predetermined period of time inorder to cure the adhesive so as to secure the gage to the diaphragm andcase. In an alternative method of making the transducer, the step ofcuring the adhesive to bond the gage to the transducer precedesinstallation of the termination board.

Other features and advantages of the invention are described or renderedobvious in the following detailed description which is to be consideredtogether with the accompanying drawings wherein:

FIG. 1 is a longitudinal section of one embodiment made in accordancewith the invention;

FIG. 2 is a longitudinal section of a modification of the embodiment ofFIG. 1;

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 1;

FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 1; and

FIG. 5 is a plan view of the strain gage employed in the embodimentsdescribed.

In the drawings, like numerals refer to like parts.

The illustrated pressure transducer is of the type which measures gagepressures, i.e., pressures which are measured with ambient pressure as areference. The transducer includes a cylindrically shaped pressurefitting 12 which is threaded at one end as shown at 14 and is providedwith a center bore 16 for introducing the fluid whose pressure is to bemeasured. The opposite end of the fitting is counterbored at 18 and isprovided with a circular peripheral flange 20 for connection to acylindrical housing or case 22.

The transducer also includes a diaphragm 32 which is of a flat circularconfiguration. The diameter of diaphragm 13 is essentially the same asthe diameter of flange 20 of fitting 12. The diaphragm can be made froma variety of materials, such as metal, quartz, reinforced plastics,graphite and sapphire. Types 17-7PH stainless steels are preferred sincethey have very high tensile strength and are highly resistant tocorrosion. Typically, the stainless steel diaphragms will have aneffective diameter of 0.5 inches and a thickness in the order of 0.005inch for a 0-15 psi transducer.

The case 22 has an internal radially directed flange 24 and its wall hasa constant internal diameter above and below flange 24. The upper andlower ends of case 22 in its as-formed state, i.e., before assembly ofthe transducer, are illustrated in dotted lines at 28 and 30respectively. The internal diameter of the case below flange 24 is sizedso that it will snugly accommodate diaphragm 32 and flange 20 of fitting12. The internal diameter of the wall of case 22 above flange 24 issized so that it will snugly accommodate a termination board 26 (thelatter is described in greater detail hereinafter). Flange 24 of thecase acts as a shoulder or stop for both fitting 12 and terminationboard 26. Preferably, but not necessarily, case 22 is formed so that itslower end 30 has an outside bevel as shown at 31. In the completedtransducer, the upper and lower ends 28 and 30 of the wall of case 22are swaged over termination board 26 and flange 20 of fitting 12 asshown at 32 and 34 in order to secure the board, diaphragm and fittingto the case.

Mounted to diaphragm 32 on the side facing termination board 26 is astrain gage identified generally by numeral 36. The gage comprisesflexible end or tab portions 42 which are not attached to the diaphragmbut instead extend up and are sandwiched between flange 24 and terminalboard 26. Gage 36 is an electrical bridge and tab portions 42 cooperatewith conductors carried by termination board 26 to couple the bridgecircuit to terminal pins 52 which form part of the terminal board.

As described in greater detail below, a flexible block 58 madepreferably of a silicone rubber is disposed over the gage. Block 58serves no particular function in the finished transducer but is used asa pressure distribution element during assembly of the transducer. Block58 covers a substantial portion of the strain gage and is provided withtwo concave sides 59 so as to leave spaces for the tabs 42 to extend upbetween flange 24 and termination board 26.

Strain gage 36 may comprise one or more strain-sensitive resistanceelements which can be formed in a variety of different configurationsdepending upon how the transducer is to be used. Thus for example, thestrain gage may comprise a single resistance element or a pair ofresistance elements which are to be connected into an exterior measuringcircuit, e.g., they may be connected as legs of an exterior bridgecircuit or to an ohmmeter. Preferably, however, gage 36 comprises aresistance bridge consisting of a plurality of interconnected resistanceelements. Preferably gage 36 is preformed as a closed bridge circuithaving terminal leads for connection to terminal pins 52 of terminalboard 26. As an alternative, gage 36 may comprise an open bridgecircuit, but a closed circuit is preferred since use of an open circuitentails an additional soldering or welding step to connect the gageresistance elements to its terminal leads. Furthermore an electricalpressure connection to a closed bridge circuit is not critical in termsof contact resistance variation an an open circuit would be.

Referring now to FIG. 5, gage 36 comprises a flexible base or carrier 38having a generally circular center portion 40 and oppositely extendingtab portions 42. A resistance bridge circuit 44 overlies and is attachedto the center portion 40 of base 38, and connected to the bridge circuitas hereinafter described so as to render it a closed bridge are fourleads in the form of conductive strips 46(a-d). The latter are arrangedin pairs and overlie and are attached to the tab portions 42. Theresistance bridge circuit and adjacent portions of conductive strips 46are encapsulated by an overlying electrically insulating layer 48(portions of the latter have been broken away in FIG. 5 for convenienceof description and illustration). Most of the area of strips 46 areexposed for electrical contact with terminal board 26. Conductive strips46 and the elements of bridge circuit 44 consist of a thin layer of aselected electrically conductive material, e.g. about 0.0002 inch thick,and may be formed in accordance with well known printed circuittechniques. Thus for example, a thin layer of metal may be deposited onthe carrier and then selected portions of the layer of metal may beetched away so as to leave a metal pattern substantially as shown inFIG. 5. Other manufacturing techniques known to persons skilled in theart also may be used to form circuit 42 and bands 44 (see H. N. Norton,Handbook of Transducers for Electronic Measuring Systems, pp. 557-564,Prentice-Hall, 1969).

Bridge circuit 44 consists of four resistors 50, 52, 54 and 56.Resistors 50 and 52 each have an end connected to lead 46a, and theiropposite ends are connected to leads 46c and 46b respectively. Resistors54 and 56 each have an end connected to lead 46d and their opposite endsare connected to leads 46c and 46b respectively.

The base 38 of strain gage 36 may be made of a variety of materials wellknown to persons skilled in the art. Polyimides are preferred as basematerials because of the flexibility of the leads, althoughnitrocellulose paper is satisfactory where the gage is used attemperatures between -100°F and +150°F. Other plastic materials also maybe used. The bridge and its leads can be made of various types ofelectrically conductive materials and preferably is made of a coppernickel alloy, such as the alloy sold under the tradename Constantan.Nickel-chromium and platinum-irridium alloys, however, have also beenfound acceptable for high-temperature applications whileiron-chromium-aluminum and iron-nickel-chromium alloys are good whenhigher gage factors are required and operating temperatures aremoderate.

A number of different materials may be used to form the insulating layer48 used to encapsulate the bridge circuit. Thus, layer 48 may alsocomprise a polyimide.

The termination board 26 comprises a circular flat electricallyinsulating disc 60 to which four electrically conductive terminal pinsor posts 52(a-d) are anchored. Affixed to the surface of board 26 whichfaces the diaphragm 32 are four electrically conductive strips 54(a-d).Posts 52 extend through disc 60 and each is electrically connected to acorresponding one of the posts 54. Strips 54a and 54b are parallel toeach other and are spaced so that they will contact the terminal leads46a and 46b respectively, when the terminal board is mounted in the case22. Similarly, strips 54c and 54d are parallel to one another and arespaced so that they will contact the terminal leads 46c and 46drespectively when board 26 is mounted on case 22.

Disc 60 is provided with one or more vent holes 62 in order that thepressure in the chamber 64 formed by board 26, case 22 and diaphragm 32is always equal to the ambient pressure. As an alternative measure asshown in FIG. 2, board 26 can be modified by eliminating vent holes 62and making one or more of the terminal posts 52 hollow (see terminalpost 54c in FIG. 2 which has a through bore 66 leading to chamber 64).

The pressure fitting 12 and case 22 of the transducer 10 can beindividually manufactured in accordance with methods well known in theart. For example, they can be produced on a screw machine, they may becold formed, forged or cast, or they can be drawn as "one hit" parts inwhich all features of the parts are formed in one stroke of a pressmachine. The case 22 should be made of a malleable material having goodsevere forming characteristics so that it can be swaged as describedbelow, while the pressure fitting 12 can be made of stiffer materials.By way of example, case 22 may be made of Type 303 stainless steel whilefitting 22 can be made of metals and alloys, e.g. a stainless steel, orplastics such as a glass reinforced epoxy.

The diaphragm 32 can be made from a number of materials such asstainless steel, beryllium, copper, glass fiber reinforced plastic,quartz, sapphire, carbon and the like, depending upon the application ofthe device. Preferably it is punched from a sheet of non-brittlematerial such as a selected stainless steel. However, if the diaphragmis made of a brittle material such as quartz or sapphire, the transducerdesign is modified as shown in FIG. 2 in order to accommodate athermoplastic ring 68 which is seated against the lower surface of theperipheral flange 20 and is clamped against the latter by the swagedover end portion 34 of case 22. Ring 68 acts as a secondary seal (inaddition to the sealant hereinafter described which is provided alongthe mating surfaces of the flange 20, diaphragm 32 and the flange 24).The primary function of ring 68 is to distribute or equalize theclamping pressure on the diaphragm so as to apply an even load along theperiphery of the diaphragm when the case is swaged in position. Ring 68permits an immense load to be put on the edge of the diaphragm,typically in the order of several thousand psi, without uneven stress.Thus, ring 68 provides a good seal and prevents fracturing of thediaphragm during manufacture. It is to be noted also that placing anuneven stress on the diaphragm can cause it to fail prematurely due tofatigue as well as affecting the accuracy of the measurements made withthe transducer. It is to be noted also that diaphragm deformation can beavoided or minimized by controlling the swaging pressure. During theswaging operation, distortion of the case (which affects the effectivediaphragm size) can be controlled and prevented by a close-fitting pinand sleeve swaging assembly (not shown) that completely surrounds andsupports the case.

In accordance with the method of this invention, the above describedtransducers can be assembled at less cost and in less time thantransducers heretofore available. The method requires less skill than isrequired with conventional methods of making strain gage pressuretransducers and can be carried out by automatic or semi-automaticassembly equipment. Furthermore, because the transducer comprises aplurality of interfitting parts, it is possible to employ diaphragmshaving predetermined characteristics such as tensile strength andflexibility.

The preferred method of assembling the transducer of FIG. 1 will now bedescribed. First of all, a liquid sealant, preferably of the anaerobicvariety, is coated on the mating surfaces of diaphragm 32, flange 24 ofcase 22, and flange 20 of fitting 12. By way of example, the sealant maybe Loctite Plastic Gasket. Then the diaphragm and fitting are insertedin the case so that the edge of the diaphragm is sandwiched betweenflanges 24 and 20 and the end 30 of case 22 is swaged over flange 20 asshown at 34 in order to clamp the diaphragm in place and trap thesealant. Preferably the clamping pressure exerted on the diaphragm is inthe order of 2000-4000 psi, which is high enough for the case, diaphragmand fitting to constitute a rigid subassembly. The latter is cleaned toremove any excess sealant.

Then the subassembly is oriented with the fitting extending down fromthe case and is axially rotated at high speed with the plane of thediaphragm kept horizontal. A metered drop of a solvent thinnedepoxy-phenolic adhesive is then dropped on the center of the top surfaceof the diaphragm. Centrifigul force causes the adhesive to spread almostinstantaneously into a thin, uniform and tack-free film. At the sametime, the upper surface of flange 24 is coated with the same adhesive.Then the strain gage 36 is inserted into the case 22 via its top end andbrought down against the diaphragm. Since the diameter of theencapsulated center portion 40 of gage 36 is only slightly smaller thanthe diameter of flange 24, the former will selflocate when placed intocontact with the diaphragm over the epoxy-phenolic adhesive. Then somemore of the same adhesive is applied to the edge of the center portionof the gage base 38, block 58 is placed over the center portion of thegage as shown, and block 58 and the gage are staked to the diaphragm bysubjecting them to spot heating at about 300°F and under about 40 psipressure. The heat staking is accomplished by engaging block 58 with aheated rod. A balancing pressure of about 40 psi pressure is applied tothe underside of the diaphragm during this staking operation to preventdiaphragm distortion. This staking operation is performed to hold thegage in place and thus only a portion of the gage is required to bebonded to the diaphragm by the staking operation.

In the preferred method of manufacture, the tab portions 42 of the gageare next spot heat staked to the adhesive coated upper surface of flange24 by heating at about 300°F under light pressure.

As an optional measure, a coating or ring of electrically insulatingmaterial (not shown) can be located between the tab portions 42 andflange 24, and/or between the ends of tab portions 42 and terminationboard 26 to further assure against the leads being shorted out by thecase. However, it is to be noted that adhesive coating on flange 24 andthe plastic substrate of the gage are non-conducting.

The termination board 26 is then inserted in case 22 so that its contactstrips 54a-d are aligned with and contact the gage leads 46a-drespectively. Preferably the board is sufficiently translucent (ortransparent) to permit visual alignment of the strips 54 and leads 46.The board 26 may also be kept in place by a friction fit with case 22 orby adhesively staking it to the case under heat and pressure. With board26 properly aligned with the tab portions of the gage, the end 28 of thecase is swaged over the edge of the board so as to form a lip as shownat 32 which holds the board and the tab portions of the gage tightlyagainst flange 24 and thereby maintains the strips 54 in permanentelectrical contact with the corresponding gage leads 46.

The resulting assembly may now be electrically tested for shorts. Theentire assembly is then placed in an oven at about 300°F forapproximately two hours in order to cure the adhesive which bonds thegage to the diaphragm and case. During this curing step, both sides ofthe diaphragm are maintained at equal pressures, preferably at about 40psi, and block 58 acts as a pressure distribution member to hold thegage flat against the diaphragm while the adhesive is being cured.Although block 58 is left in place, it serves no function in thefinished device.

Although the above method is preferred, it can be modified withoutdeparting from the invention. For example, curing of the adhesivebetween the gage and the diaphragm can be performed as a separate stepprior to installation of the termination board, in which event block 58may be removed from the case before board 26 is secured in place.

In this connection it is to be noted that it is difficult to permanentlybond silicone rubber to other materials and that in the preferred formof the method described above, the epoxy-phenolic adhesive provides abond of low peel strength between block 58 and the gage. However, thatis adequate since the only purpose of staking the block to the gage isto render it effective under applied air pressure to press the gage flatagainst the diaphragm while the intervening adhesive is being cured.Thus, the block may become separated from the gage and be loose withinthe finished device without affecting its operation or accuracy,provided however, that the block must not block off the vent holes. Thelatter problem is avoided in the embodiment of FIG. 1 by virtue of thefact that the terminal posts 52 project into the case far enough toprevent block 52 from closing off vent hole 62. If hollow terminal postsare used as in FIG. 2, the problem of vent hole blockage is avoided byproviding stand-offs or projections (not shown) on the upper side ofblock 58 that can maintain it spaced from the termination board farenough to keep passageways 66 unblocked.

It is contemplated also that block 58 may be made of other flexiblematerials such as neoprene or Buna-N rubber, other adhesives may be usedin the assembly procedure, and the gage need not be encapsulated(encapsulation is preferred since it renders the gage insensitive tomoisture). If an unencapsulated gage is used, it is preferred to makeblock 58 of neoprene and to permanently bond it to the gage by anintervening cement, whereby the block and cement effectively protect thegage against moisture. As a further option, block 58 need not beadhesively bonded to the gage, but be held in place by a friction fitwith portions of flange 24.

After assembly as above described, the complete transducer is tested tocheck on the pressure seal of the diaphragm, to insure that allelectrical connections are satisfactory and to determine overallperformance of the gage.

In operation, the pressure fitting 12 is attached to a conduit or vesselcontaining the fluid whose pressure is to be measured. one pair ofterminal posts 54 which are connected to opposite ends of the bridgecircuit, e.g. posts 54a and 54d, are coupled to a voltage source (notshown) for exciting the gage bridge. The remaining pair of terminalposts are connected to a measuring device, e.g., a meter (not shown),which measures the output voltage of the bridge. When the pressure ofthe fluid being measured is equal to the ambient pressure in chamber 64,the diaphragm will be undeflected, with the result that the gage bridgewill be balanced and the voltage output to the meter will be zero plussome offset. If, however, the pressure applied via fitting 12 is notequal to the ambient pressure, the diaphragm will deflect, producingstress and resulting strain in the diaphragm. Since the resistance armsof the gage bridge exhibit a piezoresistive effect, the resistance ofthe gage bridge arms will change in proportion to the amount of strain,causing the bridge to become unbalanced and hence produce a voltageoutput to the measuring device plus the offset which is in proportion tothe amount of strain on the diaphragm 12 and thus proportional to thepressure differential.

Since certain obvious changes may be made in the method and apparatusherein described without departing from the scope of the invention, itis intended that all matter contained herein shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:
 1. The method of making a pressure transducercomprising the steps of:providing a hollow case which is open at itsends and has an internal flange disposed intermediate said ends,inserting a diaphragm into one end of said case so that said diaphragmengages said flange, mechanically deforming the wall of said case atsaid one end so that a portion of said wall is bent over the edge ofsaid diaphragm and clamps said edge to said flange, providing a straingage comprising a main portion which includes a resistance bridge andoppositely disposed tab portions which include leads for said bridgedisposed in a predetermined geometric pattern, inserting said straingage into the other end of said case so that said main portion engagessaid diaphragm, and providing an adhesive between said main portion andsaid diaphragm, adhesively securing at least a part of said main portionto said diaphragm, providing a termination board having a plurality ofconductive segments disposed in a geometric pattern similar to that ofsaid gage leads, positioning said termination board in the other end ofsaid case so that said gage tab portions are sandwiched between saidboard and said flange and so that each of said segments is aligned andin electrical contact with the corresponding one of the leads of saidstrain gage, and mechanically deforming the wall of said case at saidother end so that a portion of said wall is bent over the edge of saidboard and clamps said board to said flange.
 2. The method of claim 1including the step of subjecting both sides of said diaphragm to equalpressures while said main portion of the gage is being adhesivelysecured to said diaphragm.
 3. The method of claim 1 including the stepof heating the assembled parts to cure the adhesive bonding the mainportion of the gage to the diaphragm after said termination board hasbeen clamped to said flange.
 4. The method of claim 1 wherein adhesiveis provided between said gage and diaphragm by dropping a limited amountof a fluid adhesive on said diaphragm at substantially the centerthereof after said diaphragm has been clamped to said flange, andrapidly rotating said case so that centrifugal force will cause saidadhesive to spread out and form a thin film covering said diaphragm.